Soft Music Keyboard/Workstation

There are lots of such products available, see for example:

And DSP hardware:

There probably won't be an exact software copy of an existing hardware synthesizer workstation though, for obvious reasons, though there's no doubt a PC could probably do all the same things. Some musicians also much prefer working with a hardware device; The MPC series hardware sampling workstations by Akai still sell plenty for the qualities of their interface even though an average PC could easily outmatch their capabilities otherwise.

There are emulators of famous keyboards of the past, but the "workstation" part (sequencing, effects, storage, etc) is usually left to the DAW software (digital audio workstation). A DAW emulates a studio recorder, a mixing console and its associated patchbay/effects. It can host the so called "plugins", specialized software modules that can emulate effects or instruments.

A fast enough single PC or Mac can be used to produce even complex tracks, keeping in mind that synthesizing and effects will require more cpu than mere sample playing, and sample based instruments will require more storage.

This one is my favorite DAW.

This explains the technology behind fx/instruments plugins.

Here are some examples of plugins in action. Most of today's music is made that way.

Drums

Acoustic piano (just to get the idea. he rocks at playing but the sound is badly recorded, the plugin actually sounds much better)

Strings and orchestra

Acoustic guitar

Electric guitar

Electric bass